The present invention relates to the formation of a cobalt/tungsten/phosphorus (CoWP) metal cap for a copper interconnect and, more particularly relates to a manufacturable process for the formation of the CoWP metal cap on such copper interconnect.
Metallization patterns on integrated circuits can be formed by depositing a dielectric layer, patterning the dielectric layer to form a trench and then depositing a metal layer that fills the trench in the dielectric layer. The metal layer typically not only fills the trenches but also covers the entire semiconductor wafer. Therefore, the excess metal is removed using either chemical-mechanical polishing (CMP) or an etchback process so that only the metal in the trenches remains.
As wire widths in integrated circuits continue to shrink, the electrical conductivity of the wiring material becomes increasingly more important. The material of choice, aluminum, is becoming less attractive than other materials, such as copper, which are better conductors. In addition to possessing superior electrical conductivity, copper is more resistant than aluminum to electromigration, a property that increases in importance as wire cross-sectional areas decrease and applied current densities increase. Copper is seen as an attractive replacement for aluminum because copper offers low cost, ease of processing, lower susceptibility to electromigration, and lower resistivity.
Copper has several serious disadvantages as a wiring material. It can diffuse rapidly into and through silicon substrates and dielectric films, such as silicon dioxide. Diffusion into an adjacent dielectric region can cause formation of a conductive path between two interconnect lines producing an electrical short. Diffusion into an adjacent silicon substrate can cause junction leakage, thereby destroying the device. Copper also has poor adhesion to the dielectric capping layer. Replacement of aluminum with copper as an interconnect material requires that these problems be overcome.
To alleviate the problem of copper adhering to the overlying dielectric capping layer, various metal caps have been proposed to cap the copper interconnect so as to increase adhesion to the overlying dielectric capping layer. One such proposed metal capping material is CoWP. Dubin et al. U.S. Pat. No. 5,695,810, Edelstein et al. U.S. Pat. No. 6,153,935 and Sambucetti et al. U.S. Pat. No. 6,323,128, the disclosures of which are incorporated by reference herein, all disclose the electroless deposition of CoWP. In Dubin et al., electrolessly plated CoWP is used as a barrier layer between the copper and the dielectric. Dubin et al. further discloses that a palladium (Pd) seed is sputtered on the dielectric for adhesion of the CoWP but no seed is deposited on the copper when the CoWP is used as a metal cap.
In Edelstein et al., a Pd seed is formed on the copper wiring material by a Pd solution of undisclosed composition prior to electroless deposition of the CoWP Residual Pd seed (not adhered to the copper) is removed by an aqueous solution of a strong complexing agent such as ethylenediamine, citrate, or ethylenediamine tetraacetic acid.
In Sambucetti et al., copper wiring material is pretreated with H2SO4 followed by treatment with a Pd solution of undisclosed composition to cause deposition of a Pd seed followed by a solution containing at least 15 g/l of sodium citrate or ethylenediamine tetraacetic acid to remove excess Pd ions. The copper wiring material is then electrolessly plated with CoWP.
Babu et al. U.S. Pat. No. 4,956,197, the disclosure of which is incorporated by reference herein, discloses the electroless plating of nickel or copper onto a dielectric substrate. The dielectric substrate is pretreated with an NH3 plasma followed by treatment with a PdCl2 seeding solution containing HCl. The PdCl2 is an extremely aggressive seeding solution and is unacceptable for the copper interconnects of the present invention.
Akai et al. U.S. Pat. No. 4,622,069, the disclosure of which is incorporated by reference herein, discloses the electroless plating of nickel or copper onto a ceramic substrate. Prior to electroless plating, the ceramic substrate was treated with an organic Pd seed solution to deposit a Pd seed.
Notwithstanding the efforts of those skilled in the art, there remains a need for a manufacturable process for depositing a CoWP capping layer for a copper interconnect.
Accordingly, it is an object of the present invention to have a process for depositing a CoWP capping layer for a copper interconnect which is well-controlled and repeatable.
It is another object of the present invention to have a process for depositing a CoWP capping layer for a copper interconnect using a Pd seed solution which enables robust seeding for CoWP while minimizing chemical attack of the copper interconnect.
These and other objects of the invention will become more apparent after referring to the following description of the invention.